The present invention relates to ignition timing control which compensates for engine knocking conditions for purposes of improving the operating efficiency of an internal combustion engine.
A number of attempts have been made to mitigate undesirable circumstances that arise out of engine knocks by controlling the engine's ignition timing by feedback using a knocking sensor as a source of a feedback parameter. The ignition timing, which is usually determined as a function of engine speed and intake airflow, is trimmed incrementally in response to a signal from the knocking sensor so that ignition occurs at an angle retarded as much as possible to meet fuel economy and high operating efficiency. Microcomputers are conveniently used for generating a basic ignition timing variable as a function of sensed engine speed and intake airflow. The basic timing variable is trimmed incrementally by a trimming value that is derived in response to the presence of an engine knock and trimmed decrementally by a trimming value that is derived in response to the absence of engine knocks, so that ignition timing is corrected in a learning process and an optimum value is thus obtained.
However, during transient periods such as rapid acceleration, engine knock tends to occur more frequently than at other times and for this reason a large amount of correction is required during transient periods. The trimming value used during such transient periods completely updates the previously learned trimming value and therefore the previous learning process becomes useless for subsequent steady state operations and a new learning process must be initiated each time the engine operation switches from transient to steady state operations. Since the learning process takes some length of time to obtain an optimum value, the engine's operating efficiency and fuel economy are not optimum during relearning periods.